This video will show you how to get all 6 hitchhikers in Wheels of Aurelia. (No hitchhikers in BRACCIANO or PIOMBINO)
This is the order they appear in the video:
1. ROMA
2. ROMA
3. CIVITAVECCHIA w/ OLGA
4. SIENA w/ OLGA
5. SIENA w/ PRIEST
6. CIVITAVECCHIA w/ GORILLA
To pull into the spots you just have to make sure you let go of the gas and you will automatically pull in.
I had my achievement unlock as soon as I pulled into the last hitchhiker, so i'm unsure if you have to finish a story for the progress to count. If anyone finds out please let me know!
Detailed descriptions:
#1 & 2 - Start from beginning (ROMA) they will both be on the way to the next turn off.
#3 - Start from CIVITAVECCHIA and choose the dialogue option "No, i'm travelling with a friend" to stay with OLGA, then grab the hitchhiker on your way to SIENA.
#4 - If you drive carefully through CIVITAVECCHIA with OLGA you will be able to get her to stay with you in SIENA, by choosing dialogue option "Your coming with me, aren't you?", where you will find the next hitchhiker.
#5 - Next you want to get to SIENA again, but this time you want to be with the PRIEST. This will happen automatically if you drive recklessly through CIVITAVECCHIA with OLGA.
#6 - Start from CIVTAVECCHIA and choose to say that you are alone so that you end up ditching OLGA and going with GORILLA through CIVITAVECCHIA to find the last hitchhiker